1. Field of the Invention
The present invention relates to a preamplifier circuit, and more particularly to preamplifier circuit that includes electrostatic discharge (ESD) protection for a magnetoresistive sensor.
2. Description of Related Art
The recording density of magnetic disk drive apparatus has increased dramatically, due to recent developments in, for example, micro-fabrication technology and control technology.
Due to the increased recording density, the magnetic domain area per unit of recorded information has been reduced, and the conventional reproducing method, which reproduces a signal by means of electromagnetic induction, using the relative motion between the head gap of a magnetic head and the magnetic domains on the disk surface, is reaching its limits. A reproducing method that employs an MR (magnetoresistive) head using the magnetoresistive effect, and does not rely directly on relative motion between the disk and the head, has therefore recently been developed.
GMR (giant magnetoresistive) heads that exploit the GMR effect have also come into practical use. These MR heads configure the MR element with layers of material such as thin metal films, helping to increase the recording density still further.
In more economical magnetic disk drives, for cost and other reasons, the head is configured to be single-ended, with one end of the MR (or GMR) head grounded to a chassis or the like of the HDD. When used in this single-ended fashion, since one end of the head is grounded to the chassis, if the chassis of the HDD acquires a high potential during assembly, for example, this potential is applied unaltered to the head.
An invention described in Unexamined Japanese Patent Application No. 169005/1995 (reference 1), for example, inserts a variable conductive device in parallel with the head; if a high voltage is applied across the MR head, the voltage is discharged through this protective element to protect the MR head. An invention described in Unexamined Japanese Patent Application No. 45033/1996 (reference 2) provides protective elements such as constant-voltage diodes, varistors, or the like between the layers of MR material constituting the MR head and the magnetic shield layers that are provided to enclose these layers of MR material, from which they are separated by insulating layers. If the voltage of the layers of MR material and the like exceeds a certain voltage, the voltage is discharged through these protective elements, so as to protect the head.
A problem with approaches in references 1 and 2, however, is that since the protective elements are disposed in parallel with the MR head, the MR head's high-frequency response is degraded by the capacitive components of the protective elements.
A further problem with the approach described in reference 2 is that if the size of the head is reduced due to increased recording density, the head becomes difficult to manufacture.
When an MR head is employed, damage to the head caused by electrostatic discharge (ESD) after the magnetic disk drive apparatus is manufactured is a rare occurrence, but when a GMR head, which has a different destruction mode from an MR head, is employed, the head is less immune than an MR head to destruction by ESD.
In view of the foregoing problems and limitations, there is a need for improved ESD protection for magnetoresistive sensors.